The present invention relates generally to filtering within a satellite communication system, and more particularly, to a method and apparatus for adjusting a notch filter frequency of a notch filter circuit within the satellite communication system.
Notch filters are known in the art and are used in various applications including the areas of servomechanisms and communication. A notch filter has a magnitude characteristic value for frequencies near zero and the same value for frequencies approaching infinity that is high relative to a specified range of frequency values, where amplitude of the notch filter is low.
Two types of notch filters exist, digital notch filters and analog notch filters. Digital notch filters can be tuned at any time but are performance limited by sample rate speed and xe2x80x9cthroughputxe2x80x9d limitations. Digital notch filters incorporate multiple analog-to-digital converters and require sampling of data. Therefore, analog notch filters are preferred over digital notch filters because of their simplicity and nonuse of sampling.
Traditionally, analog notch filters, hereinafter referred to as a notch filter, are tuned by varying and adjusting different resistors and capacitors to reach a desired notch filter frequency. The task of tuning a notch filter is tedious and time consuming. This task requires electronic unit disassembly, rework, and retest. Additionally, a notch filter frequency is not easily adjusted. Once a notch filter frequency is determined and the notch filter is constructed with the appropriate resistors and capacitors having appropriate values, the notch filter frequency is fixed and not adjustable without disassembly.
During pre-launch of a spacecraft multiple components are xe2x80x9ctweakedxe2x80x9d and modified up through the last minutes of preparation. When a notch filter frequency needs to be adjusted the notch filter is retuned as described above. The retuning potentially delays the launch and increases costs.
A typical notch filter has a first tee network on an input of the notch filter and a second tee network on a feedback path coupling an output of the notch filter to the input. Both the first tee network and the second tee network may consist of both resistors and capacitors. In order for the notch filter to operate appropriately the second tee network is required to impedance match the first tee network. The matching of the second tee network to the first tee network can be time consuming and become inconvenient especially during pre-launch of a spacecraft.
A notch filter having an adjustable notch filter frequency has been suggested by Steber and Krueger, xe2x80x9cA Voltage-Controller Tunable Active Null Networkxe2x80x9d published in proceedings of IEEE vol. 57 pp. 233-234 February 1969. The notch filter in the Steber and Krueger reference inconveniently changes gain to different values above and below the notch filter frequency as the notch filter is tuned. For example, for a specified notch filter frequency the gain for low frequencies is substantially different from the gain for high frequencies. The inconsistent gain of the notch filter is undesirable in various electrical systems as known in the art. Also, in order for the notch filter of the Steber and Kruger reference to function properly a minimum of four integrated circuits are required. The larger the number of integrated circuits the larger the cost in production of and the heavier the notch filter. Available space on a spacecraft is limited and weight of the spacecraft is a critical factor in energy consumption during launch and maneuvering of the spacecraft. Therefore, minimization of integrated circuits is preferred.
It would therefore be desirable to develop a technique for adjusting a notch filter frequency without disassembly, reworking, and retesting of a notch filter. It would also be desirable for the developed notch filter to have uniform gain for both low and high frequencies while at the same time minimizing pre-launch preparation time, size, and weight of the notch filter.
The present invention provides a method and apparatus for adjusting a notch filter frequency of a notch filter circuit within a spacecraft. A notch filter circuit is provided including a first stage circuit and a second stage circuit. The first stage circuit has a first stage input and a first stage output terminal. The first stage circuit sums a filter input signal with a filter output signal to form a summation signal. The second stage circuit includes a first tee network and a second tee network. The first tee network is electrically coupled to the first stage input and low-pass filters the filter input signal to form a first tee output signal. The second tee network is electrically coupled to the first stage output terminal and high-pass filters the summation signal to form a second tee output signal. A first multiplying digital-to-analog converter is electrically coupled to the first tee network and the second tee network. The first multiplying digital-to-analog converter modifies the gain of the second tee network in response to a gain signal to form a corrected second tee network signal, thereby adjusting a notch filter frequency. The second stage circuit sums the first tee output signal with the corrected second tee output signal to form the filter output signal. A method for performing the same is also provided.
One of several advantages of the present invention is the elimination of a tee network within a feedback path of a notch filter. In so doing, the time required to match the feedback tee with an input tee is eliminated.
Another advantage of the present invention is the ability to adjust the notch filter frequency without disassembling the notch filter circuit, thereby, saving time involved in disassembly, reworking, and retesting of the notch filter circuit.
Furthermore, the present invention allows the notch filter frequency to be adjusted remotely and during launch or while in post-launch of a spacecraft. Thus, permitting adjustments to the notch filter frequency during events when adjustments would not normally be feasible.
The ability to adjust the notch filter frequency remotely also allows a single notch filter circuit to be used for potentially multiple purposes. The notch filter circuit may have a first notch filter frequency for a first application and a second notch filter frequency for a second application.
Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.